The most expensive part of an electrostatic printer is the writing head. The most expensive part of the writing head is the high voltage driver chips that switch on the 300-600 volts needed to deposit an electrostatic charge on dielectric media that passes over the writing head.
To reduce the number of driver chips needed, many printer manufacturers commonly employ a method known as multiplexing whereby multiple numbers of nib electrodes, also known as needle electrodes or stylii, are wired in parallel. To allow firing of individual nibs, the nib voltage is set to a level, such that charge deposition only occurs when there is simultaneous firing by complementary (or plate) electrodes. These complementary electrodes are fired in a sequential pattern across the writing head, so that at any one point in time, only the nibs activated across from the energized plate electrodes are energized to write data on the dielectric media passing over the writing head. A review of the technology is provided in U.S. Pat. No. 5,515,095 (Matsuda et al.).
Unfortunately multiplexing creates problems even as it reduces the number of nib drivers. Multiplexing produces a number of undesired artifacts including various downweb (process direction) striations due to uneven crossweb charge density. Striations become noticeable after toning of the dielectric media.
There are at least three types of multiplexing striations. The most common type are "multiplexing lines" or "multiplexing striations" which are more precisely called "nib group boundary striations". These striations occur due to the fact that the printer activates the nibs in groups. U.S. Pat. No. 5,053,793 (White et al.) discloses a method for reducing nib group boundary striations.
A second type of striation associated with multiplexing is what has been termed in some companies as "plate gap striations". These are either low density or high density striations associated with the gap between two plates.
A third type of striation is what is termed a "sequence striation". This striation is again a result of the fact that the nibs are fired in groups. In this type of striation, the problem is that the effect of pulsing the plates and energizing the nibs sets up fields in the media under adjacent sets of nibs (that are not currently activated but will be activated later) that can affect the fields set up when the adjacent group of nibs is energized. The resulting striation has a variation in charge density or optical density that varies across the nib group from left to right and results in a harsh transition at the nib group boundary. It also appears that sequence striations can also be observed from nib groups that are not adjacent but two nib groups away from each other, if the time between firings of the two nib groups is not long enough.
There are several known approaches to reducing sequence striations, each with their own problems. One is to pick a sequence pattern that maximizes the amount of time between writing on nearest neighbor and 2nd nearest neighbor groups. Slowing the printer will also increase time between firings but reduce equipment productivity. Reducing the "on" time for the groups will also reduce these striations. However, this leads to reduced density and possibly increased dropouts, i.e., locations where intended imaging of data does not appear.
Another approach is identified by USSR Authorship Certificates 788069 and 1097968 where the method of line-by-line electrostatic recording of information in adjacent zones is carried out in opposite directions.
U.S. Pat. No. 5,061,948 (Hansen et al.) describes an approach to reduce sequence striations whereby the firing scheme is designed to try to balance out the field effects (or "perturbations" as described by Hansen et al.) by writing "in-between" previously written areas or in areas far enough away from previously written areas that there is essentially no field effect. In their optimized mode, Hansen et al. describe a firing scheme where the scan line is written from the center out to the edges with alternate writing on one side of the head and then the other--the so-called "Ping-Pong" approach. The resulting firing over time has a chevron, or flying "V" formation from the center to the edges. Hansen et al. realize that this approach may not eliminate all the striations since there will be some difference between groups written on a "forward step" (writing in a "unperturbed" area) versus a "return step" (writing in an area of approximately balanced perturbations). To overcome this Hansen et al. recommend leading with an "B" group instead of the usual "A" group on alternating scan lines.